Gsub and sub for replacement in Ruby - Step-by-Step Execution

Concept Flow - Gsub and sub for replacement

Start with original string

↓

Call sub or gsub method

↓

Check for first match

↓

Replace first match

↓

For gsub: Continue checking for more matches

↓

Replace all matches

↓

Return modified string

The program starts with a string, then uses sub to replace the first match or gsub to replace all matches, returning the new string.

Execution Sample

Ruby

text = "hello hello"
new_text_sub = text.sub("hello", "hi")
new_text_gsub = text.gsub("hello", "hi")
puts new_text_sub
puts new_text_gsub

This code replaces the first 'hello' with 'hi' using sub, and all 'hello's with 'hi' using gsub, then prints both results.

Execution Table

Step	Method Called	String Before	Match Found?	Action	String After	Output
1	sub	hello hello	Yes (first 'hello')	Replace first 'hello' with 'hi'	hi hello
2	puts new_text_sub	hi hello	N/A	Print string	hi hello	hi hello
3	gsub	hello hello	Yes (all 'hello's)	Replace all 'hello' with 'hi'	hi hi
4	puts new_text_gsub	hi hi	N/A	Print string	hi hi	hi hi
5	End	N/A	N/A	No more actions	N/A	Execution stops

💡 All replacements done and outputs printed, program ends.

Variable Tracker

Variable	Start	After sub	After gsub	Final
text	"hello hello"	"hello hello"	"hello hello"	"hello hello"
new_text_sub	nil	"hi hello"	"hi hello"	"hi hello"
new_text_gsub	nil	nil	"hi hi"	"hi hi"

Key Moments - 2 Insights

Why does sub only replace the first occurrence but gsub replaces all?

Because sub is designed to replace only the first match it finds (see execution_table step 1), while gsub continues to find and replace all matches (see step 3).

Does the original string change after sub or gsub?

No, the original string 'text' stays the same throughout (see variable_tracker), because sub and gsub return new strings without modifying the original.

Visual Quiz - 3 Questions

Look at the execution_table at step 1, what is the string after calling sub?

A "hi hello"

B "hello hi"

C "hi hi"

D "hello hello"

💡 Hint

Check the 'String After' column in execution_table row for step 1.

At which step does gsub replace all occurrences?

A Step 1

B Step 2

C Step 3

D Step 4

💡 Hint

Look for the row where Method Called is 'gsub' and action mentions replacing all matches.

If we used sub instead of gsub in step 3, what would be the output at step 4?

A "hello hello"

B "hi hello"

C "hi hi"

D "hello hi"

💡 Hint

Recall sub replaces only the first match, so only the first 'hello' changes.

Concept Snapshot

sub and gsub replace text in strings.
sub replaces only the first match.
gsub replaces all matches.
Both return new strings; original stays unchanged.
Use sub for single replacement, gsub for global.

Full Transcript

This visual execution shows how Ruby's sub and gsub methods work for replacing text in strings. Starting with the string 'hello hello', sub replaces only the first 'hello' with 'hi', resulting in 'hi hello'. Then gsub replaces all 'hello' occurrences with 'hi', resulting in 'hi hi'. The original string remains unchanged throughout. The execution table tracks each step, showing method calls, matches found, actions taken, and resulting strings. Variable tracking confirms that the original string 'text' does not change, while new variables hold the replaced strings. Key moments clarify why sub replaces only the first match and gsub replaces all, and that original strings are not modified. The quiz tests understanding of these steps and outcomes.

Practice

1. What is the main difference between sub and gsub methods in Ruby?

easy

A. sub replaces all matches, gsub replaces only the first match.

B. sub replaces only the first match, gsub replaces all matches.

C. sub deletes matches, gsub adds new text.

D. sub works only on arrays, gsub works on strings.

Solution

1. Step 1: Understand sub behavior

   sub replaces only the first occurrence of the pattern in the string.

2. Step 2: Understand gsub behavior

   gsub replaces all occurrences of the pattern in the string.

3. Final Answer:

   sub replaces only the first match, gsub replaces all matches. -> Option B

4. Quick Check:

   sub = first match, gsub = all matches [OK]

Hint: Remember: sub = single replace, gsub = global replace [OK]

Common Mistakes:

• Confusing which method replaces all matches
• Thinking sub works on arrays
• Believing gsub replaces only one match

2. Which of the following is the correct syntax to replace the first occurrence of 'cat' with 'dog' in the string "cat and cat" using sub?

easy

A. "cat and cat".substitute('cat', 'dog')

B. "cat and cat".gsub('cat', 'dog')

C. "cat and cat".replace('cat', 'dog')

D. "cat and cat".sub('cat', 'dog')

Solution

1. Step 1: Identify correct method for first occurrence replacement

   sub replaces only the first match, so it is the correct method here.

2. Step 2: Check method syntax

   The syntax is string.sub(pattern, replacement), so "cat and cat".sub('cat', 'dog') is correct.

3. Final Answer:

   "cat and cat".sub('cat', 'dog') -> Option D

4. Quick Check:

   sub syntax = string.sub(pattern, replacement) [OK]

Hint: Use sub for first match: string.sub('old', 'new') [OK]

Common Mistakes:

• Using gsub instead of sub for first occurrence
• Using non-existent methods like replace or substitute
• Wrong method call syntax

3. What will be the output of the following Ruby code?

text = "hello hello hello"
result = text.sub('hello', 'hi')
puts result

medium

A. hi hello hello

B. hello hi hello

C. hi hi hi

D. hello hello hi

Solution

1. Step 1: Understand what sub does here

   sub replaces only the first occurrence of 'hello' with 'hi'.

2. Step 2: Apply replacement to the string

   The first 'hello' becomes 'hi', the rest remain unchanged, so the string becomes "hi hello hello".

3. Final Answer:

   hi hello hello -> Option A

4. Quick Check:

   sub replaces first match only [OK]

Hint: sub changes only first match, others stay same [OK]

Common Mistakes:

• Thinking sub replaces all matches
• Confusing output with gsub behavior
• Misreading the string content

4. The following code is intended to replace all occurrences of 'apple' with 'orange' in the string, but it only replaces the first one. What is the error?

fruit = "apple banana apple grape"
new_fruit = fruit.sub('apple', 'orange')
puts new_fruit

medium

A. Use gsub instead of sub to replace all occurrences.

B. The string variable should be an array for replacement.

C. The replacement string must be in double quotes.

D. The method sub does not exist in Ruby.

Solution

1. Step 1: Identify the method used

   The code uses sub, which replaces only the first match.

2. Step 2: Correct method for replacing all matches

   To replace all occurrences, gsub should be used instead of sub.

3. Final Answer:

   Use gsub instead of sub to replace all occurrences. -> Option A

4. Quick Check:

   gsub replaces all matches, sub replaces first only [OK]

Hint: Use gsub for all replacements, sub for first only [OK]

Common Mistakes:

• Using sub when gsub is needed
• Thinking sub works on arrays
• Believing method names are case sensitive differently

5. Given the string "2023-04-01 and 2023-05-01", you want to replace only the month part (the two digits after the first dash) with '12' for all dates. Which Ruby code correctly does this using gsub and a regex?

hard

A. "2023-04-01 and 2023-05-01".sub(/-(\d{2})-/, '-12-')

B. "2023-04-01 and 2023-05-01".gsub(/\d{2}-/, '12-')

C. "2023-04-01 and 2023-05-01".gsub(/-(\d{2})-/, '-12-')

D. "2023-04-01 and 2023-05-01".gsub(/-\d{2}/, '-12')

Solution

1. Step 1: Identify the pattern to replace

   The month is the two digits between dashes, so the regex -(\d{2})- matches the dash, two digits, then dash.

2. Step 2: Use gsub to replace all matches

   Using gsub with the pattern and replacement '-12-' replaces the month part in all dates.

3. Final Answer:

   "2023-04-01 and 2023-05-01".gsub(/-(\d{2})-/, '-12-') -> Option C

4. Quick Check:

   gsub with regex replaces all month parts correctly [OK]

Hint: Use gsub with regex matching month pattern for all replacements [OK]

Common Mistakes:

• Using sub instead of gsub for all matches
• Incorrect regex missing dashes
• Replacing wrong parts of the string